SAMUT SONGKRAM, Thailand (AP) – Rushing across a temple parking lot, British angler Rick Humphreys yells, “We’ve got a fish.”

He jumps into a small motorboat on the Maeklong River in time to see Wirat Moungnum bring the prize to the surface – a rare, giant freshwater stingray.

It bursts through the murky water exposing a soft, white underbelly the size of a trash can lid. The crew scrambles to string a rope through its gill-like slits and wrap a towel around its 5-foot-long tail that has a venomous barb.

“It’s a start,” Humphreys says almost apologetically. The specimen is a tenth of the size of the largest rays. “There are a lot bigger ones than that.”

Humphreys is serving as a guide for American biologist Zeb Hogan, who is on a worldwide quest for the largest freshwater fish.

Hogan, 34, has heard the stories of Cambodian fishermen catching rays that weighed over 1,100 pounds with wingspans of 14 feet. But so far, they are just stories. If he can confirm them, he could eclipse the world record now held by the Mekong giant catfish.

“It could be the largest fish in the world and we know next to nothing about it,” Hogan says. “I’ve spent five years on the Mekong looking for rays and only saw two or three. They were nowhere near the size I’d heard about.”

Hogan’s quest is part of the Megafishes project financed by the National Geographic Society.

The three-year project, which started in 2006, aims to document and protect freshwater giants that weigh at least 200 pounds or measure 6 feet long. The project will take Hogan to 14 freshwater systems on six continents, including the Mekong, Nile, Mississippi and Amazon rivers.

Time is running out for many of the species. The Chinese paddle fish and the dog-eating catfish in Southeast Asia are on the brink of extinction because of pollution, overfishing and dam building.

“Of the two dozen or so species of giant fish, about 70 percent are threatened with extinction,” says Hogan, an assistant research professor at the University of Nevada-Reno.